This invention is directed to a method and apparatus for locating faults in electric cables, in particular the high impedance or nonlinear impedance faults in enclosed cables, such as buried or pipe cables.
Locating a cable fault is often a time-consuming and laborious process, the ultimate cost of which is highly dependent on the accuracy that can be achieved in the initial measurement of the distance from the cable terminals to the fault. The measurement errors inherent in the presently available equipment make necessary some type of tracing technique to determine the exact location of the fault. Any improvement in the accuracy of the measurement of the fault distance from the terminal will reduce the cost by reducing the time consumed in finding the exact location with tracing techniques. This is particularly important since the number, voltage rating, and length of electric cables installed by U.S. utilities is rapidly increasing.
Furthermore, the efficiency of existing measurement equipment is dependent on the nature of the fault, particularly its impedance. For faults with impedance of 50 ohms of less, existing equipment is considered satisfactory. High-impedance faults, which often occur after an initial breakdown of the cable insulation, require that some impedance reduction technique be used. As the power ratings of cables increase, the size and cost of the equipment required to perform this also increases. A more accurate, convenient method of fault location is clearly needed. This method should utilize a single instrument capable of supporting both terminal and tracing techniques through a simple procedure adaptable to all types of faults.
The accuracy of available fault location equipment is limited by several factors associated with both the cable and the measurement instrument itself. These uncertainties reduce the accuracy with which the distance from the fault to the cable terminals can be measured and makes tracing techniques necessary. Tracing is often a time consuming and laborious process, so that improved fault location equipment has the potential for substantially reducing the cost of locating faults in electric cables.